Re: [PATCH RFC 1/1] ASoC: fsl_ssi: Make fifo watermark and maxburst settings device tree options

From: Caleb Crome
Date: Thu Jan 14 2016 - 16:26:48 EST

On Thu, Jan 14, 2016 at 12:18 PM, Nicolin Chen <nicoleotsuka@xxxxxxxxx> wrote:
> On Thu, Jan 14, 2016 at 08:29:42AM -0800, Caleb Crome wrote:
>> Tuning the SSI fifo watermark & maxburst settings needs to be
>> optimized differently depending on the demands on the system. The
>> current default of 2 is too low for high data-rate systems. This
>> patch maintains exactly the same behavior by default (i.e defaults to
>> 2), but adds device tree options to set maxburst & fifo depth from the
>> device tree. This is necessary because a setting of 2 simply doesn't
>> work at higher data rates.
>> @@ -61,6 +61,16 @@ Optional properties:
>> - fsl,mode: The operating mode for the AC97 interface only.
>> "ac97-slave" - AC97 mode, SSI is clock slave
>> "ac97-master" - AC97 mode, SSI is clock master
>> +- fsl,fifo-watermark: Sets the fifo watermark. The default is
>> + fifo_depth-2 words, meaning 'initiate dma transfer
>> + when 2 words are left in the fifo'. At higher
>> + data rates (48kHz, 16-channels for example), this
>> + causes silent but deadly DMA xruns and channel
>> + slips. For 15 word FIFOs (like on MX5, MX6) 8 is
>> + a good value when running at high data rates
>> +- fsl,dma-maxburst: sets the max number of words to transfer in DMA.
>> + This defaults to the same value as
>> + fsl,fifo-watermark.
> I think DT maintainers may not give a consent towards these two
> properties as they are not to describe the hardware but to hack
> software configurations. (And it seems you haven't CCed them.)

Yeah, I thought I'd just ask alsa first, rather than send to DT
maintainers. Is it preferable to just send to everybody that
get_maintainers spits out even for an RFC?

> I forgot which values you've figured out for these two properties,
> but I think those two values should work for normal cases as well:
> as SSI only has limited FIFO depth, it won't hurt (increasing too
> much latency) even if using a higher watermark configuration imo.
> So it could be a good idea to use optimized settings for all use
> cases and let other users test it.
> Nicolin

As for optimal settings, I finally came to a setting of 4 for depth &
maxburst, which will result in more DMA requests, but it's the only
way that works at 48kHz for me. The default settings is 13 (15 - 2)
for the ones of the 15 item fifo, which is a pretty dramatic
difference. I just don't know if other chips will behave badly in
that case.

I'd be happy to just submit a patch that sets it to 4 if we think
that's the right way to go.